The present invention relates to calcined kaolin clay. In a more specific aspect, the present invention relates to a process for compacting calcined kaolin clay.
Kaolin is a naturally occurring, relatively fine, white clay mineral which may be generally described as a hydrated aluminum silicate. After purification and beneficiation, kaolin is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The use of kaolin clay in paper coatings serves, for example, to improve brightness, color, gloss, smoothness, opacity, printability and uniformity of appearance of the coated paper. As a filler in paper formulations, kaolin clay is used to extend fiber and reduce cost and to improve opacity, brightness and other desirable characteristics of the paper product.
Calcined kaolin clay is a particular type of kaolin and is often used in large quantities for paper manufacture. Calcined kaolin can be obtained by heating beneficiated kaolin clay at temperatures of at least 550xc2x0 C. The calcination step dehydroxylates and converts the kaolin to a noncrystalline aluminosilicate phase. The term xe2x80x9cdehydroxylatesxe2x80x9d refers to the removal of structural hydroxide groups from the kaolin as water vapor. Calcined kaolin has improved light scattering characteristics (as compared to the non-calcined kaolin) and, therefore, contributes a high degree of opacity to the coated paper.
Fanselow et al. U.S. Pat. No. 3,586,523 describes examples of calcined kaolin clay.
Calcined kaolin clay pigments (such as the product marketed by Thiele Kaolin Company of Sandersville, Ga. under the trademark KAOCAL) are widely used in the paper industry. Commonly, calcined clay is blended with hydrous kaolin in the paper mills prior to the manufacture of a finished paper product. These blends are typically comprised of 10-30 parts by weight calcined kaolin and 70-90 parts by weight hydrous kaolin.
During processing, the calcined clay is usually pulverized in a high energy impact mill and then air-classified to remove abrasive particles larger than 325 mesh as these particles tend to cause scratching problems during coating of a paper. The resulting pulverized calcined clay product is fluffy and has a low bulk density when compared to a spray dried hydrous kaolin clay. For example, the bulk density of calcined kaolin is about 10-15 lb/ft3, whereas the bulk density of spray dried hydrous kaolin is about 40-55 lb/ft3.
The fluffy low bulk density calcined clay product can be difficult to handle with conventional bulk handling systems. Therefore, the product is shipped dry in bags, sparger cars (i.e., freight cars) or as an optimally dispersed slurry, typically at about 50% solids. In addition, because the low bulk density of dry calcined clay will commonly require either larger bags or larger volume freight cars when compared to a spray dried hydrous clay, the dry calcined clay is more expensive to ship.
In the industry, efforts have been made to improve the bulk density, dusting and powder flow characteristics of calcined clay. For example, in Suitch et al. U.S. Pat. Nos. 5,074,475 and 5,129,953, an aqueous slurry of calcined clay is spray dried to improve bulk density and flowability. However, this spray dried material tends to create dust and can be difficult to handle in bulk and is expensive to dry due to the energy costs associated with spray drying.
Cook et al. U.S. Pat. Nos. 4,561,597 and 4,593,860 increase the bulk density of calcined clay by dry ball milling calcined clay powder, but the flowability of the powder remains poor. Furthermore, the dry ball milling step needs to be followed by pulverizing in a high-energy impact mill to minimize slurry screen residue. However, pulverizing after the dry ball milling step may also result in a product with a lower bulk density.
Dunaway et al. U.S. Pat. No. 5,364,579 pelletizes calcined clay using a pan pelletizer or pin mixer in the presence of 1% ammonia vapor to improve flowability. This process does not use water and, therefore, eliminates the use of an energy intensive drying step. However, the Dunaway et al. patent does not disclose any improvements in bulk density or integrity of the pellets compared to the original feed calcined clay.
Crumbley et al. U.S. Pat. No. 5,328,506 discloses a process in which calcined clay is kneaded to form dough-like agglomerates using 1-30% moisture. The claimed advantages of this process are that the product is shipped without drying and is dust free, resists break down to dust during shipping and is capable of being dispersed in water when agitated using conventional make down equipment. However, this process does not use any pressure for agglomeration, and consequently the agglomerated moistened calcined clay product even when combined with up to 30% hydrous clay does not show an improved bulk density.
Munsterman et al. International Patent Publication No. WO 97/42268 describes a process for producing a free flowing, dust free and high bulk density material using a mixture of water (20-48 percent) and calcined clay, followed by pelletizing in a disc pelletizer and then drying to a moisture content of 2 percent. The bulk density of the agglomerated product is reported to be about 30 lb/ft3. The disc pelletizer forms rounded agglomerates by tumbling action, and no external pressure is applied. Consequently, the resulting agglomerated product is expected to be relatively weak. More importantly, the production costs of this process are high due to the expense of the drying step.
A process for the manufacture of high bulk density agglomerates from a mixture of calcined kaolin clay, hydrous kaolin clay and water is disclosed in Maxwell et al. U.S. Pat. No. 6,238,473. In this process, the mixture is subjected to high pressure in a compaction system.
Consequently, there is a need in the industry for a process by which calcined clay can be compacted to provide a high bulk density product which can be packaged in bulk for handling and transporting in conventional systems and which has low screen residue, while still retaining the optical properties which are desirable for paper coating and filling applications, but without the addition of either water or hydrous kaolin clays.
Briefly described, the present invention provides a process for compacting calcined kaolin clay. The compacted clay of this invention allows for optimal or maximum weight loading of conventional shipping containers and exhibits properties which are either equivalent to or improved over the corresponding properties of the starting non-compacted calcined kaolin clay.
Accordingly, an object of this invention is to provide a process for compacting calcined kaolin clay.
Another object of this invention is to provide a process for compacting calcined kaolin clay in which the compacted clay has a high bulk density.
Another object of this invention is to provide a process for compacting calcined kaolin clay in which the compacted clay has improved wettability.
Another object of this invention is to provide a process for compacting calcined kaolin clay in which the compacted clay has an improved time for slurry incorporation.
Another object of this invention is to provide a process for compacting calcined kaolin clay in which the compacted clay can be packaged for optimal or maximum weight loading of a conventional shipping container.
Still another object of this invention is to provide a process for compacting calcined kaolin clay in which the compacted clay has a low 325 screen mesh residue.
Still another object of this invention is to provide a process for compacting calcined kaolin clay in which the compacted clay retains good rheological properties.
Still another object of this invention is to provide a process for compacting calcined kaolin clay in which the compacted clay retains good optical properties.
These and other objects, features and advantages of this invention will become apparent from the following detailed description.